fix(compiler-cli): don't generate empty <target/> when extracting xliff

[ocombe]

PR Type

What kind of change does this PR introduce?

[x] Feature

What is the current behavior?

When we extract xliff files with ng-xi18n, it creates empty <target/> elements in the extracted file. Xliff is the only format with that behavior.
It's especially problematic for xliff because the same file can be used as source and as translation without changing the content (which is not the case for xmb/xtb that use a different markup). When using the extracted file as a translation file without changing its content, all traductions are empty and they don't trigger the missingTranslation parameter.

Issue Number: #15754

What is the new behavior?

We don't add empty <target/> elements to the extracted file anymore.

Does this PR introduce a breaking change?

[x] No

[mary-poppins]

You can preview 4ae5a4a at https://pr18072-4ae5a4a.ngbuilds.io/.

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